There are many applications where an object must be positioned in multiple degrees of freedom, for example six degrees of freedom, with high precision in the nanometer range, while also being movable, generally in one or two such degrees of freedom, over a greater range, for example 200 to 300 millimeters with, for example, 10 nm accuracy Such applications might include scanned probe microscopy; however, the primary application would be in precision, mechanically suspended linear slides in XY stages, such as those used in the motion control subsystem of a photolithographic machine for producing semiconductor integrated circuits.
Current wafer stepping machines use compound axes in coarse/fine stages to achieve travels of about 200 mm in X and Y with resolution better than 100 nm. The camera head may be moved on flexures to provide Z-axis focusing motion. Such devices are relatively large and heavy and achieve positioning in six degrees of freedom through use of numerous actuators, including rack and pinion or ball screws for the coarse motion and piezoelectric or miniature hydraulic actuators for the fine motion. Thus, the overall system is complex and is also very expensive. It is also difficult to design each stage to be free of resonances, and thus to provide fast settling times as the stage moves from one chip site to another.
In order to reduce complexity both in the system itself and in the design thereof, magnetically-suspended XY stages have been proposed for such applications. Copending application Ser. No. 632,965, filed Dec. 20, 1990, teaches a magnetic bearing which may be utilized for maintaining the position of an object with a high degree of precision and for making small position adjustments which would typically not exceed 250 microns in any direction. Such a stage could therefore be used only for very fine positioning, and one or more additional coarse positioning stages would be required to achieve the degree of movement required for XY positioning applications such as in photolithographic machines for semiconductor fabrication.
A need therefore exists for an improved positioning device which would provide positioning control in the 10 nm range, preferably in six degrees of freedom, while permitting movement with good acceleration and settling time over a range of several hundred mm, for example 200 to 300 mm, preferably in both the X and Y direction. Such magnetic positioning device might also provide the capability for controlled movement in the Z direction (i.e. in a direction perpendicular to a work surface), also with precision in the 10 nm range.
While many magnetic linear positioning devices are described in the literature, most of these devices provide for motion in only a single direction. Further, such devices employ toothed magnetic elements and/or toothed or slotted electromagnetic actuators. This results in a cogging when no actuating current is applied to the device, or, in other words, in detenting occurring at certain preferred positions. This means that the only way a precision position can be maintained which is not one of the detent positions is to maintain current in the coils, which current must be sufficient to overcome the detent effect This cogging effect thus makes it far more difficult to achieve precise positioning with fine resolution, makes it harder to maintain stability of the device at a precisely determined position, and increases the time required to stabilize the device at a desired position. Teeth on the magnets and/or on the coils are therefore undesirable.
Further, where coils are in slots or on an iron core, as is the case for most prior art linear actuators, the device has a narrower frequency response, and also has a non-linear hysteresis curve which makes the device harder to control and results in some energy losses. Such hysteresis losses, in conjunction with eddy currents which also exist in such cores, reduce the high frequency response and power efficiency of the device. All of this results in poor stiffness for the device, or in other words, in decreased stability.
Existing devices also normally operate in a stepping mode. However, there are applications, for example in semiconductor fabrication, where a scanning mode of operation is desirable wherein movement from a first point to a second point is accomplished at a precisely controlled speed. This permits exposure to be performed along a strip which allows a simpler optical design in the exposing lens.
A need therefore exists for an improved positioning device which is preferably capable of positioning an object in the X and Y degrees of freedom with travel in the 200 to 300 mm range with 10 nm resolution, and with good acceleration and stabilization times. The device should also be capable of maintaining a desired position for the object in six degrees of freedom with the same level of resolution, should be of lower cost than existing systems and should permit operation in either a stepping mode or a scanning mode.